1. Field of the Invention
This invention relates to the lens auto-focusing system of a digital still camera, which automatically switches the lens focusing point between two zones, namely from near zone to far zone or reverse way according to the signals of the distance judgments made by and sent from the CPU or electronically controlled distance measuring system of the camera.
2. Background Art
In the conventional auto-focusing of digital still cameras, the lens is focused at the object by checking the image formed on the image sensor like CCD sensor or C-MOS sensor that is controlled by the CPU (Central Processing Unit) provided in the camera. First the lens starts moving from its home position (standing position) and moves back and forth, and while the lens is moving, frequently the CPU checks the image formed on the image sensor like CCD or C-MOS through the lens, and then the lens stops at the position where the image becomes most sharply in focus. In this case the lens must be driven back and forth in straight succession along the ring cam, which is provided with the lens driving mechanism and is rotated by a stepping motor so that the lens may move back and forth when the stepping motor is rotating. While the lens moves back and forth, the CPU detects the moment when the image becomes most sharply in focus, and at that moment of the best image detected the CPU sends a signal to the lens to stop moving. Then the lens stops at that position, where the image is in best focus. After capturing a picture, the lens must return to its home position (standing position), and to return the lens to the home position the ring cam starts rotation again. When the lens comes back to the home position, the photo-interrupter provided beside the lens barrel detects the home position of the lens with a kind of shutter provided on the rim of the lens barrel, which shuts the light of the penetrating type photo-interrupter. And the moment when the photo-interrupter detects the home position, the lens moving stops at that position, namely at the home position.
Thus in the ordinary case of auto-focusing system of digital still cameras, the lens must be driven in straight succession, and the image detection must be done in frequent succession, and the lens must be returned to its home position each time after capturing a picture. These steps of automatic focus setting of the lens take time and in many cases miss the best picture capturing chances. Further more the mechanism that consists of a stepping motor, a ring cam and a photo-interrupter is rather expensive in cost.
FIGS. 1 and 2 show a typical conventional auto-focusing system for achieving the heretofore explained functions. When the stepping motor (1) receives a signal to start from the CPU of the camera, the stepping motor starts rotating and the stepping motor gears (2) that are engaged with the ring cam gear (3) provided on the rim of ring cam (4) starts rotating. Then the ring cam (4) starts rotating with its ring cam gear (3) engaged with the stepping motor gears (2), and the position sensing rod (5) starts moving back and forth (up and down) along the aslope surface of ring cam (4), then the whole lens assembly (6) starts moving back and forth along the guide rail (7) which is provided with the lens assembly (6). The one side of the lens barrel is shaped like a sleeve, and on the sleeve the position-sensing rod (5) is provided as a part of the sleeve protruding from the sleeve, and on the outermost part of the sleeve of the lens assembly (6) the axle bearing part (8) is provided, which moves along the guide rail (7) back and forth. On the other side of the lens barrel there the position-stabilizing fork (9) is formed as a part of the lens barrel, and between the ends of the fork (slot) the position-stabilizing pin (10) is provided.
The axle bearing part (8) and the guide rail (7) support the whole lens assembly (6) together with the position-stabilizing fork (9) and position-stabilizing pin (10). The lens assembly (6) is pressed always toward the image sensor (I 1) by the coil spring (I 2) so that the tip of position sensing rod (5) always touches the aslope surface of the ring cam (4) properly without fail.
While the power switch of the digital still camera is on, an image is formed on the image sensor (11) through the lens. The instant that the switch of auto-focusing system is turned on, the focusing status of the image is to be sent to the CPU to check the image formed on the image sensor in frequent succession, and the instant that the image comes to the sharpest status in focus, the CPU gives a signal to stop the checking, namely to get the stepping motor (1) to stop. Then the shutter can be released and the image is captured. After these steps, the lens assembly (6) must be returned to its home position (standing position). On the rim of the ring cam (4), the position sensing shutter-blade (13) is provided. After capturing a picture, the ring cam (4) starts rotating again to return the lens assembly (6) to its home position, and when the position-sensing shutter-blade (13) comes into the slot of photo-interrupter (14), the light (beam) of the photo-interrupter (14) is shut off by the position-sensing shutter-blade (13), and the ring cam (4) stops at once there, so that the lens assembly (6) may return and stop at its home position. The FIG. 2 shows the lens assembly being at its home position.
As explained in this example, it is necessary for the lens assembly (6) to move back and forth continuously to let the CPU check the image status. Such searching of the best focusing point by repeating image checking with mechanical and continuous lens movement takes time inevitably, and may cause users to miss good shutter chances. Further more the mechanism itself is rather complicated, and must be delicately adjusted, and assembling of the mechanism is difficult. The parts and components used in the mechanism are rather expensive. So it is preferable to have a much simpler mechanism with much less and inexpensive parts and components, and with much less processing time for autofocusing.